1. Field of the Invention
The present invention relates to a structure of a bonding apparatus and to a method for adjusting the height of a bonding stage in the bonding apparatus.
2. Description of the Related Art
In semiconductor manufacturing processes, wire-bonding apparatuses are used for bonding of wires to pads on a semiconductor chip mounted on a circuit board and leads on the circuit board for connection therebetween. Many wire-bonding apparatuses include a bonding arm configured to be rotated by a drive motor about a rotation center and to vertically drive a bonding tool, such as a capillary, attached at a tip end thereof and, through the operation of the bonding arm, bond and connect a wire inserted through a tip end of the bonding tool to pads and/or leads (see FIG. 1 of Japanese Patent No. 3683801, for example).
Such bonding apparatuses require positioning of the tip end of the bonding tool over pads and/or leads for bonding, for which image-based positioning methods are becoming common. Japanese Unexamined Patent Application Publication No. Sho 53-25356, for example, proposes a method in which a TV camera is arranged right over a mark on a work placed on a table, and the positional relationship between the capillary and the work is determined with the mark appearing on the monitor of the TV camera being aligned with a reference on the monitor and, based on this determination, bonding is performed. Japanese Unexamined Patent Application Publication No. Hei 8-111430 also discloses an arrangement in which a certain region on a semiconductor chip and tip end portions of leads are imaged by imaging means such as an ITV camera, the amount of displacement of an imaging target is obtained with respect to a reference coordinate (image center) on the image, and a correction is introduced based on the amount of displacement to recognize the positions of pads on the semiconductor chip and the tip end portions of the leads. In this arrangement, the recognized positions of the pads and the tip end portions of the leads are stored in storage means prior to wire bonding and, based on this stored data, the tip end of the bonding tool is repositioned in the bonding apparatus.
In contrast, since high accuracy is required for pad position detection, bonding apparatuses configured to keep the occurrence of position measurement errors as low as possible are becoming common with an arrangement in which the rotation center of the bonding arm has the same height as the surface of a lead frame on which a semiconductor chip for bonding is mounted, and the motion trajectory of the tip end of the bonding tool is made perpendicular to the surface of the lead frame during bonding as well as imaging means is arranged perpendicular to the surfaces of the lead frame and the semiconductor chip.
A bonding apparatus 100 according to a related art will hereinafter be described with reference to FIGS. 10 to 13. As shown in FIG. 10, the bonding apparatus 100 according to the related art includes: a bonding stage 113 for holding thereon a lead frame 112 on which a semiconductor chip 141 is mounted; a reference plane 111 parallel to the surface 114 of the bonding stage 113; a bonding arm 121 configured to be rotated by a drive motor 117 about a rotation center 115 that lies in the reference plane 111 and to move a capillary 123 attached at a tip end thereof in a direction toward and away from the lead frame 112, which is held on the bonding stage 113 with the surface thereof having the same height as the reference plane 111, and the semiconductor chip 141 that is mounted on the lead frame 112; and an imaging device 125 for imaging the surfaces of the semiconductor chip 141 and the lead frame 112 to detect the positions of pads provided on the semiconductor chip 141 or leads provided on the lead frame 112. In FIG. 10, the bonding stage surface 114 and the reference plane 111 are in parallel with the XY plane, a horizontal plane, with the Z direction in the figure pointing upward with respect to the bonding apparatus 100. The rotation center 115 of the bonding arm 121 is shown as an intersection point between the reference plane 111 and a centerline 132 extending in the Z direction.
The rotation center 115 of the bonding arm 121 is provided at the same height as the reference plane 111 and the surface of the lead frame 112 above the bonding stage surface 114. Consequently, the tip end of the capillary 123 moves on a motion trajectory 135, a circular arc that intersects perpendicularly with the reference plane 111 and the surface of the lead frame 112. The imaging device 125 is provided in such a manner that an optical axis 151 thereof is made perpendicular to the reference plane 111. It is noted that the optical axis 151 runs through the center of the imaging device 125 to serve as a reference when detecting the positions of pads and/or leads. The imaging device 125 is also provided with a predetermined spacing from the capillary 123 and the bonding arm 121 in the X direction in FIG. 10.
FIG. 11 illustrates the plane of rotation of the bonding arm 121 and the plane including the optical axis 151 of the imaging device, which are arranged with a predetermined spacing in the X direction, on the same plane. As shown in FIG. 11, the optical axis 151, which serves as a measurement line of the imaging device 125, is a straight line perpendicular to the reference plane 111, while the motion trajectory 135 of the tip end of the capillary 123 is a circular arc that intersects perpendicularly with the reference plane 111 at point A. Therefore, the optical axis 151 and the motion trajectory 135 become misaligned in the Y direction with increasing distance from the reference plane 111. This misalignment in the Y direction then emerges as a Y-direction position measurement error by the imaging device 125. The amount of this misalignment in the Y direction between the optical axis 151 and the motion trajectory 135 can be calculated using the radius r1 of the bonding arm 121 and the position of the rotation center 115. As indicated by the curve “t” in FIG. 13, the amount of misalignment in the Y direction increases against the distance from the reference plane 111 as the tip end of the capillary 123 moves away from the reference plane 111, but if the rotation center 115 of the bonding arm 121 lies in the same plane as the reference plane 111, the amount of misalignment in the Y direction is 2 μm to 3 μm at the largest against the distance from the reference plane 111.
In the bonding apparatus 100 illustrated in FIGS. 10 and 11, the bonding arm 121 is moved downward, prior to bonding operations, such that the tip end of the capillary 123 comes into contact with the surface 140a of the semiconductor chip 140 that is mounted on the lead frame 112 and, with the downward movement being halted, the angle of rotation of the bonding arm 121 is measured to determine the height H0 of the surface 140a of the semiconductor chip 140 for bonding and calculate the Y-direction position of point C0 on the motion trajectory 135 at the height H0, thereby to obtain the difference between the Y-direction position of C0 and the Y-direction position of point B0 on the optical axis 151 at the height H0 as the amount of misalignment Δ0 in the Y direction between the optical axis 151 and the motion trajectory 135 of the tip end of the capillary 123. The amount of misalignment Δ0 in the Y direction is then set as a Y-direction offset for values measured by the imaging device 125 to correct the amount of misalignment in the Y direction between the straight optical axis 151 and the circular motion trajectory 135.
The height of the surface of the semiconductor chip for bonding from the reference plane 111 is known to vary within the range of several tens of micrometers due to a variation in the thickness of the lead frame 112 on which the semiconductor chip is mounted or the joint thickness when the semiconductor chip is mounted on the lead frame 112. For example, as shown in FIG. 11, when the height of the surface 141a of the semiconductor chip 141, which is mounted on the lead frame 112, from the reference plane 111 changes from the original height H0 of the surface 140a of the semiconductor chip 140 to H1, the amount of misalignment in the Y direction between the optical axis 151 and the motion trajectory 135 changes from the offset amount of misalignment Δ0 in the Y direction to the difference Δ1 in the Y-direction position between points B1 and C1 at the height H1, with a difference d1 therebetween, as shown in FIG. 13. If the difference in the height between the surfaces 140a and 141a of the respective semiconductor chips 140 and 141, i.e., (H1−H0) is several tens of micrometers, the difference d1 is within the range from 0.1 μm to 0.2 μm, which has little effect on bonding accuracy.
However, in the above-described bonding apparatus according to the related art, the rotation center 115 of the bonding arm 121 is required to have the same height as the reference plane 111, as shown in FIGS. 10 and 11, to improve position measurement accuracy. This requires the rotation center 115 to be provided above the bonding stage 113 to prevent interference between the bonding arm 121 and the bonding stage 113. In this case, increasing the size of the bonding stage 113 and thereby widening the bonding area so as to support bonding for larger semiconductor chip 141 would require an increase in the length of the bonding arm 121. However, increasing the length of the bonding arm 121 would also result in an increase in the weight and therefore the moment of inertia thereof, suffering from a problem in that the bonding cannot be sped up.
Hence, it can be considered, as shown in FIG. 12, to provide the rotation center 215 of the bonding arm 221 above the reference plane 211. FIG. 12 illustrates the plane of rotation of the bonding arm 221 and the plane including the optical axis 251 of the imaging device, which are arranged with a predetermined spacing in the X direction, on the same plane, as is the case in FIG. 11. In this case, the motion trajectory 235 of the tip end of the capillary 223 is a circular arc with a radius r2 centering on the rotation center 215 above the reference plane 211, the circular arc intersecting with the reference plane 211 at point A not perpendicularly but obliquely, as shown in FIG. 12. For this reason, the amount of misalignment in the Y direction between the optical axis 251 and the motion trajectory 235 against the height from the reference plane 211 becomes much larger compared to the case shown in FIG. 11 where the motion trajectory 135 intersects perpendicularly with the reference plane 111. The amount of this misalignment in the Y direction can also be calculated using the radius r2 of the bonding arm 221 and the position of the rotation center 215, as indicated by the curve “u” in FIG. 13.
As is the case in the bonding apparatus 100 described with reference to FIGS. 10 and 11, also in the bonding apparatus 200 illustrated in FIG. 12, the bonding arm 221 is moved downward, prior to bonding operations, such that the tip end of the capillary 223 comes into contact with the surface 240a of the semiconductor chip 240 that is mounted on the lead frame 212 and, with the downward movement being halted, the angle of rotation of the bonding arm 221 is measured to determine the height H0 of the surface 240a of the semiconductor chip 240 for bonding and calculate the Y-direction position of point C′0 on the motion trajectory 235 at the height H0, thereby to obtain the difference between the position of C′0 and the Y-direction position of point B′0 on the optical axis 251 at the height H0 as the amount of misalignment Δ′0 in the Y direction between the optical axis 251 and the motion trajectory 235 of the tip end of the capillary 223. The amount of misalignment Δ′0 in the Y direction is then set as a Y-direction offset for values measured by the imaging device 225 to correct the amount of misalignment in the Y direction between the straight optical axis 251 and the circular motion trajectory 235.
Whereas, as shown in FIG. 12, when the height of the surface 241a of the semiconductor chip 241, which is mounted on the lead frame 212, from the reference plane 211 changes from the original height H0 of the surface 240a of the semiconductor chip 240 to H1, the amount of misalignment in the Y direction between the optical axis 251 and the motion trajectory 235 changes from the offset amount of misalignment Δ′0 in the Y direction to the difference Δ′1 in the Y-direction position between points C′1 and B′1, with a difference d′1 therebetween greater than d1 above, as shown in FIG. 13. If the difference in the height between the surfaces 240a and 241a of the respective semiconductor chips 240 and 241, i.e., (H1−H0) is several tens of micrometers, the difference d′1 reaches as high as 6 μm to 7 μm, which is a non-negligible error in bonding accuracy. This further constitutes a major bonding problem if the variation in the height of semiconductor chips is large as in, for example, multi-layer semiconductors in common use recently.
In order to address this problem, there has been proposed a method in which a bonding arm is suspended by a link so that a virtual point of rotation lies on the surface 214 of the bonding stage (see Japanese Patent No. 3683801, for example). However, link-based arrangements of this type generally have a complex structure and large weight, with a possibility of falling of foreign objects from the structures.